1. Field of the Invention
The present invention relates to the field of image compression, and in particular to a method and apparatus for block based image compression with multiple non-uniform block encodings.
2. Background Art
Typically, there is a desire to compress images used for texturing operations in 3D graphics accelerator hardware to improve the performance of rendering and reduce the memory required to store the information. However, prior art image compression methods frequently produce undesired visual artifacts when rendering images, making their application appropriate for a limited number of images. This problem can be better understood with a review of image compression.
Image Compression
Prior art compression techniques can be categorized in at least two ways. First, a technique can be either loss-less or lossy. Second, a technique can be either fixed or variable rate. A loss-less compression technique is one in which no information about the image is lost due to the compression. Thus, an image could be compressed and decompressed and the decompressed image would be identical to the original image. A lossy compression technique is one in which some information about the image is lost due to the compression. Thus, compression of an image followed by decompression could result in a decompressed image that is not identical to the original image.
A fixed rate compression technique reduces the storage requirement of an image by a fixed percentage. Since most image formats do not contain unnecessary data, almost all fixed rate compression techniques are lossy in general (however, they may not be lossy for a particular image). A variable rate compression technique reduces the storage requirement of an image by an amount that is not known at the time when the image is compressed. In fact, the storage requirement may not be reduced at all.
Because loss-less, variable rate compression techniques are unable to guarantee any rate of compression, graphics systems typically use lossy, fixed rate compression techniques. However, the lossy nature sometimes results in unacceptable image quality (i.e., visual artifacts). Sometimes, it is so difficult to determine when compression will result in unacceptable loss of image quality and when it will not that users and/or systems elect to forgo compression altogether. As systems are developed that utilize larger amounts of information about the system, the desire for compression increases, but the problem of unacceptable image quality loss also increases in prior art compression systems.